This invention relates generally to analog integrated circuits, and more particularly to current sources implemented in analog integrated circuits.
Constant current sources and constant voltage sources are used for a variety of purposes in analog integrated circuits. As used herein, xe2x80x9cconstantxe2x80x9d means that the output of the source remains at a relatively constant direct current (d.c.) level, although the output levels of such sources can typically be adjusted (xe2x80x9csetxe2x80x9d) with a control signal. Once set, the output of a constant current or voltage source may change with temperature (i.e. be xe2x80x9ctemperature dependentxe2x80x9d) or may be stable with temperature. In many applications, it is desirable to have a constant current or voltage source that does not vary in output as the temperature changes. If the output of a constant, temperature stable, current source is coupled from an output resistor that is temperature stable, the result is a constant, temperature stable voltage source, as will be appreciated by those skilled in the art. These constant, temperature stable voltage sources are useful for many purposes, such as providing a reference voltage, for adjusting the threshold of a comparator, etc. that are useful power supplies for charging a battery
Unfortunately, however, conventional battery chargers are not capable of charging batteries that have been discharged below approximately 2.5 to 3.0 volts. One approach to charging batteries below 2.5 to 3.0 volts to a range of approximately 1.0 volt is described in U.S. Pat. 6,225,787 B1 issued to Chen et. al. entitled xe2x80x9cTEMPERATURE STABILIZED CONSTANT CURRENT SOURCE SUITABLE FOR CHARGING A HIGHLY DISCHARGED BATTERYxe2x80x9d. Unfortunately, the approach described in the ""787 patent is incapable of charging batteries that are fully depleted, i.e., in the range of 0.0 volts since the temperature stabilized constant current source described therein requires at least 1.0 volt battery voltage in order to generate an appropriate and sufficient battery charging current
Therefore, what is desired is a temperature stabilized, adjustable, yet constant current source suitably arranged to charge a battery that has been discharged to as low as 0.0 volts.
The invention is an electrical circuit that provides a temperature stabilized current source with a stable control voltage capable of charging a battery having been discharged to as low as approximately 0 volt. As used herein, xe2x80x9cstablexe2x80x9d means that the voltage remains essentially unchanged with changes in temperature, i.e. it is not temperature dependent. The circuit solves the problem of providing an adjustable temperature stabilized current source suitable for charging fully discharged (i.e. in the range of 0.0V) batteries.
The temperature stabilized, constant current source battery charger suitable for charging a fully discharged battery includes a current based feedback control circuit responsive to small signal changes in a battery charging current wherein the feedback control circuit maintains the battery charging current within a predetermined range of current values at a particular operating temperature. A temperature compensation circuit coupled from the feedback control circuit that is responsive to a temperature change having a sensing resistor of a given resistor technology coupled from at least one positive temperature coefficient voltage source and at least one negative temperature coefficient current source arranged to provide said sensing resistor with a temperature stabilized control current such that temperature dependencies of the negative temperature coefficient current source substantially countervails the at least one positive temperature coefficient voltage source such that a sense voltage developed by the sensing resistor is substantially constant over a predetermined operating range of temperatures. The current source further includes a bias resister coupled to the temperature compensation circuit arranged to provide a bias current to the temperature compensation circuit based upon a supply voltage such that the temperature compensation circuit is operative when the battery is fully discharged.
As a method for providing a temperature independent current suitable for charging a fully discharged battery, a battery charging current is maintained to the battery within a specified range of battery charging currents using a feedback control circuit and a bias resistor. The feedback controlled battery charging current is further temperature stabilized over a range of operating temperatures by a temperature stabilized current source having positive temperature coefficient current source and a countervailing negative temperature coefficient current source that maintains a temperature stabilized current through a sensing resistor. The bias resistor provides a bias current based upon a supply voltage to the temperature stabilized current source so as to provide a battery charging current to a fully discharged battery.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions of the invention and a study of the several figures of the drawing.